1. Field of the Invention
The present invention relates to an amplifier circuit and a transceiver, and more particularly, is preferably applied to, for example, a portable telephone.
2. Description of the Related Art
A portable telephone system represented by a personal digital cellular (PDC) and a personal handyphone system (PHS) is widely used. Various systems are proposed as the communication systems of these types of portable telephones. The time-division multiple access (TDMA) system is a typical system.
The TDMA system realizes a plurality of communications (so-called multiplex communication) with channels having the same frequency by temporally dividing one frequency channel so as to efficiently use frequencies. A portable telephone using the TDMA system uses a time-division separation system referred to as a time division duplex (TDD) system as a system for separating a transmission signal from a reception signal in which transmission and reception are respectively performed in separate time.
Moreover, the code division multiple access (CDMA) system has been recently proposed as a communication system having a frequency utilization efficiency higher than that of the TDMA system. The CDMA system realizes multiplex communication with one frequency channel by assigning a different diffusion code to each user and by multiplying the diffusion code by transmission data.
In such a portable telephone using the CDMA system, an inter-modulation distortion that does not occur in the above TDD system may occur because transmission and reception are performed at the same time. The inter-modulation distortion occurs because a transmission signal leaks into a reception system and serves as an interference wave and it is mixed with an original reception signal. Hereafter, the occurrence theory of the inter-modulation distortion is specifically described below by referring to FIG. 1 showing an antenna peripheral circuit of a portable telephone.
In the portable telephone 1, a transmission signal S1 amplified by a transmission amplifier circuit 2 is supplied to an antenna 4 through a duplexer 3 at the time of transmission and a radio wave corresponding to the transmission signal S1 is emitted to the space by an antenna 4. On the other hand, at the time of reception, in the portable telephone 1, a reception signal S2 received by the antenna 4 is amplified by a reception amplifier circuit 5 through the duplexer 3 and thereafter, supplied to a demodulation circuit (not illustrated) and etc. at the rear stage.
In this case, the duplexer 3 serves as a circuit for separating the transmission signal Si from the reception signal S2. However, because a transmission power is usually much larger than a reception power, the separation by the duplexer 3 cannot be completely performed and thereby, the transmission signal S1 may leak into a reception system. When the transmission signal S1 leaking into the reception system and the reception signal S2 received by the antenna 4 are inputted to the reception amplifier circuit 5, an inter-modulation distortion occurs due to the nonlinear characteristic of the reception amplifier circuit 5. When the transmission signal S1 serving as an interference wave has a frequency equal to the frequency of the reception signal S2 being a desired wave, the inter-modulation distortion serves as an interference wave to deteriorate the signal-to-noise ratio (S/N ratio). Consequently, the inter-modulation distortion deteriorates the reception sensitivity of the portable telephone 1.
To reduce the inter-modulation distortion, a method is considered which improves the separation degree of the duplexer 3 or the linearity of the reception amplifier circuit 5. When adopting either method, the size of a transistor or current consumption must be increased. Consequently, power consumption is increased.
To make it possible to use the portable telephone 1 driven by a battery for a long time, it is necessary to reduce the power consumption of the portable telephone 1. The above described reception amplifier circuit 5 is operated even under a standby state. Therefore, if the power consumption of the reception amplifier circuit 5 can be reduced, it may be possible to reduce the entire power consumption of the portable telephone 1. Therefore, in the reception amplifier circuit 5, it is preferable to minimize the power consumption even when an inter-modulation distortion is prevented.
In general, when the portable telephone 1 has a large transmission power and a small reception power, the reception sensitivity is deteriorated due to an inter-modulation distortion. Therefore, in the portable telephone, by increasing the current consumption of the reception amplifier circuit 5 only when an inter-modulation distortion influences the reception sensitivity, it may be possible to prevent the reception sensitivity from deteriorating while preventing the power consumption from increasing. In the above portable telephone, when, for example, the reception amplifier circuit 5 has a current consumption of 16 mA and an inter-modulation distortion of xe2x88x92104.6 dBm, the inter-modulation distortion becomes xe2x88x92119 dBm by increasing the current consumption to 54 mA and thus, it is possible to reduce the inter-modulation distortion by 14.4 dBm. However, even in this case, the current consumption is increased approx. three times. Consequently, such a problem occurs that the power consumption is increased to shorten the continuous working time of a battery.
In view of the foregoing, an object of this invention is to provide an amplifier circuit and a transceiver capable of preventing the reception sensitivity from deteriorating without increasing power consumption.
The foregoing object and other objects of the invention have been achieved by the provision of an amplifier circuit provided at the first stage of a reception circuit of a transceiver having a transmission circuit and the reception circuit. The reception circuit comprises an amplification element for amplifying a reception signal input through an input terminal and outputting it to an external unit from an output terminal, an adjustment circuit connected between the input terminal and output terminal of the amplification element and constituted by connecting a phase rotation element and a resistive element in series, and a load element connected to the output terminal of the amplification element, so as to eliminate an inter-modulation distortion caused by a transmission signal leaking from the transmission circuit, by setting the rotation angle of the phase rotation element and the resistance value of the resistive element to desired values.
By connecting the adjustment circuit, which is constituted by connecting a phase rotation element and a resistive element in series, between the input terminal and output terminal of an amplification element and by setting the rotation angle of the phase rotation element and the resistance value of the resistive element to desired values, it is possible to eliminate an inter-modulation distortion caused by a transmission signal leaking from a transmission circuit without increasing the current output from the output terminal of the amplification element and thereby, reduce the inter-modulation distortion while preventing current consumption from increasing.
Moreover, the present invention provides a transceiver comprising a transmission circuit and a reception circuit. The reception circuit is provided, at the first stage, with an amplification element for amplifying a reception signal input through an input terminal and outputting it to an external unit from an output terminal, an adjustment circuit connected between the input terminal and output terminal of the amplification element and constituted by connecting a phase rotation element and a resistive element in series, and a load element connected to the output terminal of the amplification element, and moreover provided with an amplifier circuit for eliminating an inter-modulation distortion caused by a transmission signal leaking from the transmission circuit by setting the rotation angle of the phase rotation element and the resistance value of the resistive element to desired values.
By connecting the adjustment circuit, which is constituted by connecting the phase rotation element and the resistive element in series, between the input terminal and output terminal of an amplification element and by setting the rotation angle of the phase rotation element and the resistance value of the resistive element to desired values, it is possible to eliminate an inter-modulation distortion caused by a transmission signal leaking from a transmission circuit without increasing the current output from the output terminal of the amplification element and thereby, reduce the inter-modulation distortion while preventing current consumption from increasing.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.